The various monomethylated tetralins are among a host of solvents which may be used as a desorbent in separations, such as those using a simulated moving bed, such as Sorbex.TM. separations technology (see, for example, Handbook of Petroleum Refining Processes, edited by Robert A. Meyers, 8-80 et ff., McGraw-Hill Book Company (1986)). However, the full exploitation of these methyltetralins is currently limited by their availability. It also has been found that the desorption properties of the 5- and 6-methyltetralins are quite sensitive to components other than the methyltetralins. Therefore it is necessary not only to prepare the requisite materials in commercial quantities, but it is also necessary that their preparation be relatively inexpensive, and that any method used leads, at the very least, to mixtures of the monomethyltetralins containing little polyalkylated material or other components deleterious to the use of the monomethyltetralins as desorbents.
Bouncer, EP 160145A, has taught that tetralin can be selectively monoalkylated with olefins using as catalysts a wide pore amorphous silica-alumina. Muganlinsk et al., SU1076424-A have obtained [presumably] monoalkyltetralins in 95-98% yield by alkylating tetralins with aliphatic alcohols containing 4 to 10 carbon atoms using a sulfated silica-alumina (approximately 7:1) containing iron oxide, calcium oxide, and magnesium oxide. However, it seems likely that alkylation occurs via olefin arising from the alcohol, since olefinic oligomers are the principal side products of the reaction. The patentee in J81009958-B used a 7:1 silica-alumina in the alkylation of tetralin with propylene to give 6-isopropyltetralin in 92% yield. More recently Innes et al., U.S. Pat. No. 4,891,458, have used zeolite .beta. to alkylate aromatic hydrocarbons with olefins of 2-4 carbon atoms in the liquid phase, claiming that zeolite .beta. has a higher selectivity to the monoalkyl products and a longer catalyst lifetime than other zeolites.
Preparation of the monomethyltetralins can not be effected by olefinic alkylation. Furthermore, methylation using methanol as the alkylating agent is notoriously more difficult than analogous alkylations using higher alcohols. Consequently, the prior art is of limited value in identifying satisfactory processes and catalysts for the preparation of monomethyltetralins. In addition, any process is subject to a number of constraints dictated by the economy of methyltetralin production and by the required product purity. In particular, it is necessary to have the conversion of tetralin in the methylation reaction as high as possible to maximize product formation. Conversions of tetralin (as defined within) of greater than 95% are ideal, although conversions as low as about 65% are acceptable if compensated by other factors. Another limitation is that the selectivity of 5- and 6-methyltetralin production should be at least 65%. Selectivity is defined as the percentage of reaction product which is 5- and 6-methyltetralin. The reaction product consists not only of alkylated tetralins, but also of unidentified decomposition products of tetralin itself. The latter are particularly irksome and deleterious, for they give the reaction product an objectionable color, removable if at all only with great difficulty, which may seriously impair the acceptability of the monomethyltetralins as desorbents, and which also may impair their desorbent properties.
We have found a regime of alkylation conditions which produces methyltetralin with a selectivity of at least 65%, usually at least 75%, at a tetralin conversion of at least 65%, often greater than 80%, and occasionally greater than 90%. Because tetralin decomposition seems to be an unavoidable concomitant of alkylation, and because tetralin decomposition products are deleterious to the contemplated use of the 5- and 6-methyltetralins, the reaction conditions are further circumscribed by the requirement that there be less than 120% conversion of tetralin. The acceptable reaction regime encompasses a relatively narrow range of tetralin to methanol proportions, a relatively narrow selection of solid acidic catalysts, and a limited temperature range which is a complicated, often puzzling function of all of the foregoing.
Although we have found that certain silica-aluminas may be used as catalysts in our process, we have determined that zeolite .beta. is a superior catalyst. Its superiority is associated with the fact that it is a significantly more active catalyst than the silica-aluminas while being equally selective. This means that the selective alkylation of tetralin with methanol can be performed with zeolite .beta. at a significantly lower temperature than that possible with the silica-aluminas. There are several attendant beneficial consequences to this greater activity. One is that the reaction can be run to high conversion with high selectivity and with very little tetralin decomposition, thereby affording a higher quality product. Secondly, because the product quality is less sensitive to temperature, the overall process is not so sensitive to operator or process control error. Additionally, as the catalyst becomes deactivated there is a greater range over which the temperature may be raised to compensate for catalyst deactivation before reactant decomposition becomes an issue. Because zeolite .beta. is more active than the silica-aluminas, less of zeolite .beta. is required to catalyze the reaction at a given production rate, thereby enhancing process economics. These advantages are substantial and may be unique to zeolite .beta..